patch 8.2.2880: unified diff fails if actually used

Problem:    Unified diff fails if actually used.
Solution:   Invoke :diffupdate in the test.  Fix the check for working external
            diff. (Ghjuvan Lacambre, Christian Brabandt, closes #8197)
diff --git a/src/version.c b/src/version.c
index 796899b..1c4bbfa 100644
--- a/src/version.c
+++ b/src/version.c
@@ -751,6 +751,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    2880,
+/**/
     2879,
 /**/
     2878,